To start this quest you talk to tylda, she can be found in the town near the blacksmith with the broom. William can be found in monestry as one of the recruits. Beppo is one of bandits in the swamp, you can find him near the entrance to the main temple. Leon is hiding in an abandoned cabin in the woods, you can find him by going in the direction to the town gate (Not the one controlled by the inquisition) and turning the nearest left
turn after crossing the temple near the road to the monestry.